Add error handling for not found changesets from GitHub

Created by: eseliger

This fixes the implementation of the ChangesetSource interface, which states a LoadChangeset implementation should throw a ChangesetNotFoundError. It also introduces a new operation to the reconciler, which tracks a changeset is initially imported and not just synced, in which case it won't go to the errored state, but remain unsynced and in a terminal error state, so no retries are attempted.

This fixes https://github.com/sourcegraph/sourcegraph/issues/14923